1. Field of the Invention
The present invention relates to a vertical diode geometry and method for fabricating the diode. More specifically, the invention relates to a vertical diode geometry with integrated bottomside ohmic contact that is bonded to a host substrate and a method for fabricating the diode.
2. Description of Related Art Including Information Disclosed Under 37 CFR §§ 1.97 and 1.98
Schottky diodes are critical and ubiquitous devices used for high-frequency electronics. These devices have been used for decades as detectors for measuring signal power level at millimeter and terahertz frequencies, as mixers in heterodyne receiver instruments, and as variable capacitors in frequency multipliers. Each of these components is a building block for the instrumentation used in radio astronomy, radar and imaging systems, as well as many other applications.
The success of the Schottky diode has largely been due to the development of processing methods that have allowed it to evolve from a vertically-oriented whisker-contacted device (that can be unreliable and is difficult to integrate into circuits) to a laterally-oriented planar device that is compatible with modern IC circuit design and processing. The ability to integrate the diode into planar circuitry allows the implementation of sophisticated circuit architectures that yield improved performance. In addition, the adoption of wafer-bonding methods that permit diodes to be attached to alternative substrate materials with superior mechanical strength and thermal properties has led to diode-based integrated circuits with unprecedented levels of performance. This approach to diode processing and design is the key technology behind the success of several recent start-up companies marketing millimeter and submillimeter-wave instruments.
While the advantage of laterally-oriented (planar) diodes lies in their compatibility with planar IC processing technology, the planar orientation results in a diode ohmic contact that is coplanar with the diode anode contact. This orientation requires current to travel parallel to the substrate surface and results in significant series resistance due to (1) the practical distance at which the ohmic contact can be fabricated, (2) skin effect and current crowding at the surface, and (3) trapping or other imperfections associated with the semiconductor-air interface. As a consequence, the intrinsic performance of planar devices has not exceeded that of the best vertical whisker contacted devices, which typically can exhibit significantly lower series resistances and parasitic capacitance.
Originally, submillimeter-wave Schottky diodes were realized using a wire whisker contacting the surface of a semiconducting substrate (such as GaAs, see FIG. 1(a)). This approach to fabricating diodes resulted in the highest-frequency and lowest-parasitic diodes ever realized. However, difficulties associated with the mechanical robustness of the whisker contact as well as integrating the device into sophisticated circuit architectures led to the development of a “planar” or lateral embodiment in which the diode contact was defined lithographically with anode and ohmic contact oriented parallel to the semiconductor substrate (FIG. 1(b)). Loss, fragility, and poor thermal properties of the semiconductor (GaAs) substrate led to the development of substrate replacement methods in which the diode material was bonded to a separate substrate with desirable properties (such as quartz). An example of this basic concept, shown in FIG. 1(c), is presently the approach used for commercial diode-based instruments.
Alternative diode architectures designed to improve the performance of the device while permitting compatibility with standard planar circuit layouts include the GaAs membrane diode developed at the Jet Propulsion Laboratory (FIG. 2) and the quasi-vertical diode concept explored by the Institut fir Hochfrequenztechnik, Technische Hochschule in Darmstadt, Germany (FIG. 3). The JPL approach has permitted realization of diode circuits to 2 THz and beyond, but the GaAs membrane is fragile and constitutes a poor thermal conductor. The quasi-vertical diode geometry from Darmstadt is an attempt to create a diode that combines the advantages of the whisker contacted diodes (low resistance and parasitics—see FIG. 4) and planar diodes (compatibility with IC fabrication and integration).